The present invention relates generally to a system for the activation of precious metal containing monomers or comonomer precursors utilizing plasma polymerization techniques. The monomer or comonomer precursors are positioned within the glow zone for conversion to the dissociated form wherein the precursors are enriched with radicals, electrons and ions (i.e., plasma conversion), and deposited as a platinized coating. The platinized coating is further characterized by its being deposited as a plasma polymer or copolymer onto and/or into an appropriate substrate also disposed within the glow zone. Although a variety of substrates are useful, the plasma polymerization operation also preferably includes utilization of a porous electrically insulative substrate such as one consisting of glass. In accordance with the present invention, the entire process including each of the individual operations is undertaken at low temperatures, and specifically at or about room temperature. The features of the present invention permit these operations to be undertaken without need to heat either the source percursor or the substrates employed in the process.
Presently known methods utilized for making films of noble metals such as platinum, gold platinum, ruthenium and alloys such as platinum and palladium, or other precious metals or alloys including for example alloys of platinum/copper and platinum/tin do not appropriately and completely cover the entire surface of a porous structure including porous membranes, beads, aerogels, fibers, porous silicon, or others. The present invention is distinguishable inasmuch as it utilizes chemical vapor infiltration followed by low temperature, low energy plasma induced metallization for the application of continuous layers or films of precious metals and/or alloys. Conventional vapor state plasma discharge operations, for example, has been found to be ineffective for pore penetration of the films, and in particular is ineffective for use with a variety of platinum compounds. Additionally, the size of the particulate obtained from conventional vapor state plasma fails to be generally in nanometer dimensions.
In accordance with the present invention, an electrical discharge from an RF generator is applied to the external electrodes of a capacitively coupled tubular plasma reactor. The articles are initially pre-exposed to the vapors. Thereafter, the selected monomer or comonomer precursors are introduced along with a carrier gas into the reactor and energized into a plasma. The area of highest plasma energy density within the reactor is controllable, and typically controllably disposed in the area between the electrodes and the plasma glow zone.
While positioned within the plasma glow zone, the substrate is preferably rotated in order to allow the substrate to receive a uniform deposit of plasma polymer. As an alternate procedure, the electrodes may be moved to accommodate unusual configurations, such as elongated substrates or the like. In accordance with the present invention, it is not necessary that components be held at elevated temperatures, and hence the operations may proceed at low temperatures, such as at or about room temperature.
A preferred material for use in connection with the present invention is platinum (II) hexafluoroacetylacetonate. The hexafluoroacetylacetonate compound is commercially available and is further identified through CAS #65353-51-7 FW=609.22.
One system for the continuous production of the platinized glass substrate employs an RF plasma reactor of tubular configuration. This reactor employs a pair of capacitively coupled external electrodes positioned at either end of the reactor, and is externally coupled to an RF generator. The highest energy density is maintained in the area between the electrodes, that is, the plasma glow zone, by controlling both the current from the RF generator, the gas supporting the plasma, and optionally, the monomer or comonomer flow rate. In this situation, when the flow rate becomes too rapid, the glow zone will "spill over" to the region outside of the electrodes. On the other hand, if the flow rate is too slow, the plasma will fail to ignite or will fail to fill the entire inter-electrode region. The chambers employed are in vacuum-sealed relationship, and each is provided with an outlet to a vacuum pump. The reactor chamber may be formed of any material with sufficient resistance to withstand the plasma polymerization reaction condition. Preferably materials found suitable for this application are quartz, Pyrex.TM. and Vycor.TM.. In addition, certain plastics have been found satisfactory as well as various ceramics.
Proper spacing between the electrodes in the plasma reactor depend upon the size of the reactor tube. One system which has been found useful employs electrodes approximately 10-15 centimeters apart, with the tube having a diameter of 25 mm. When larger diameter tubes are employed, the energy density associated with the plasma glow zone should be maintained as closely as possible to that in the smaller tubular reactor. Along with energy density, it is extremely important to control the density of the monomer as well as the density of the plasma carrier gas. In this instance, the monomer is platinum (II) hexafluoroacetylacetonate. Although monomer density will generally remain the same with changes in tube size, some variation in the optimum of both energy density and monomer density will result with changes in system size and design. In the system described herein, the current from the RF generator is preferably maintained at an appropriate level for the application. Depending upon the size and the configuration, those of skill in the art are able to ascertain the appropriate power level. Other conditions are as follows: EQU Frequency=13.56 Megahertz.
This frequency is appropriate for this application, and is authorized for use by the Federal Communications Commission. Other frequencies can be employed, particularly lower frequencies in the kilohertz range.
Methods for plasma generation between electrodes using an electric field are well known in the art. A DC field, or an AC field from 50 Hz to about 10 GHz are typical. Power values ranging from between about 1 watt to 5,000 watts are suitable.
A preferred electrical field generating means for plasma processing is the use of a high frequency power supply to initiate and sustain the plasma. The preferred operating frequency is 13.56 MHz. Other frequencies, such as 75 KHz are sometimes employed. The particular frequency and power values chosen depend on the deposition requirements of the coating materials and substrates.
Also well known in the art are potentially beneficial modifying means for increasing the ionization potential and/or providing improved spatial control of the plasma through the use of separate magnetic fields, i.e., electron cyclotron resonance (ECR) microwave plasma technique.
A useful guide in determining changes in reaction parameters with changes in tubular geometry is the composite discharge parameter W/FM, where "W" is the plasma wattage, "F" is the flow rate of the monomer or monomers, and "M" is the molecular weight of the monomer or monomers. As a tubular geometry and system size vary, W/FM may vary for a given plasma polymer or copolymer deposition rate, but optimum W/FM will vary between one-half to twice that of the original W/FM for a given monomer system. Therefore, for a given monomer system, changes in the composition plasma parameter with changes in tubular geometry may be expressed: EQU (1/2W.sub.a /F.sub.a M.sub.a &lt;W.sub.b F.sub.b M.sub.b &lt;(2)W.sub.a /F.sub.a M.sub.a
where W.sub.b /F.sub.b M.sub.b is the complete plasma parameter for a first tubular RF tubular reactor as described herein, and where W.sub.a /F.sub.a M.sub.a is the composite plasma parameter for a differently sized tubular reactor as described herein.
While this reactor design has been found useful, other designs will, of course, be capable of application to the process as well.
The arrangement can readily be adapted to continuous production or treatment of flat films, discs, rods, tubes, filaments, beads, or other articles by providing a suitable reactor chamber with supply and take-up spools or fixtures to accommodate the workpieces.